Abradable seals are used in turbomachinery to maintain the closest clearances between spinning blades and the surrounding case structure. They are comprised of materials which are particularly adapted to fragment and disappear when contacted by the blade tips which are spinning at high speeds. As generally mentioned in U.S. Pat. Nos. 3,879,831 to Rigney et al and 3,084,064 to Couden et al, abradable seals must have a peculiar combination of properties. On the one hand they must be resistant to erosion from the high velocity gas streams which at times carry fine particulate matter with them. On the other hand, they must be capable of disintegrating when contacted by the tip of a high speed blade, so that the tip of the blade is not substantially degraded. This mode of behavior is highly desirable because if the rotating parts and surrounding casing come together too closely at one point around the circumference, it is in the casing that the wear should take place. When this is not so and when the blade tips wear, then the clearance between the blades and case will be increased all around the circumference, with the result that the leakage is greatly increased beyond that which would occur with a properly functioning abradable seal.
The foregoing is only a simplified description of the properties which abradable seal must have. It must also of course be structurally sound to resist failure at points other than where the blade is contacting it, it must resist the thermal and vibratory strains imposed on it by its use, it must be readily fabricated in a reproducible and cost effective manner, and so forth. Considerable effort has gone into the development of abradable seals which have the desired combination of properties and the present invention is reflective of that continuing effort.
In the past, abradable materials have been comprised of heterogeneous combinations of non-metals and metals, as described in the patents mentioned above; or, as porous structures, obtained by use of a fugitive material in the precursor article. See U.S. Pat. No. 4,269,903 to Clingman et al and U.S. Pat. No. 3,540,884 to Horbury.
The present invention is concerned with the last mentioned process, and particularly with seals which are used in the compressor part of a gas turbine engine. Typically, compressor blades are made out of titanium, nickel or iron base alloys. Operating temperatures range from a few hundred degrees up to 540.degree. C. The abradable seal material is generally applied to a substrate structure, most commonly a circumferential ring attached to the casing. In the prior art, pressing and sintering and other powder metallurgical techniques have been used together with thermal spraying to make porous structures. Metals can be sprayed with densities as low as 75-80% by plasma arc spraying them alone, with the correct set of parameters. However, to obtain densities less than this, which are found to be desirable for abradable seals, it is necessary to incorporate a non-metallic material. Most preferably a fugitive material such as a water soluble salt or a volatilizable polymer is sprayed with the metal and removed in a second operation. For instance, an abradable seal structure can be made by spraying a polyester resin and a nichrome metal, generally along the lines taught by Longo et al in U.S. Pat. No. 3,723,165, and then removing the polyester by heating the resultant structure to a temperature of about 540.degree. C. in air.
However, the metal abradable seals made previously have not exhibited a fully satisfactory set of properties. In some instances, the abradability will be good but the mechanical strength, and in particular the erosion resistance, will be inadequate. When parametric and material changes are made to increase the latter property, then the abradability is insufficient and excessive blade wear takes place. Because of the complex dynamics of engine operations and the high cost of testing materials by running them in an actual engine, the simplified specimen testing which has been employed has not lead to the easy identification of materials which are good and materials which are inferior.